Process for removing butenes from C4-hydrocarbon streams and two-stage evaporation

Abstract

A process for removing butenes from C4-hydrocarbon streams containing butanes and butenes involves extractive distillation with a suitable solvent. The process also involves heat integration, which allows utilization of the heat of the solvent for heating and/or at least partly evaporating various streams.

The invention claimed is: 
     
         1 . A process for separating butenes from a C4-hydrocarbon stream which contains at least butenes, n-butane, and isobutane, by extractive distillation with a solvent, the process comprising:
 a. at least partially evaporating a liquid C4-hydrocarbon stream in a feed evaporator, to supply an absorber with a gaseous C4-hydrocarbon stream, supplying the solvent to the absorber above the gaseous C4-hydrocarbon stream and contacting therein the gaseous C4-hydrocarbon stream and the solvent with one another to transfer predominantly butenes from the gaseous C4-hydrocarbon stream to the solvent, to obtain a laden solvent, wherein the laden solvent is collected in a liquid collector and subjected to a two-stage evaporation in a first evaporator and a second evaporator to outgas predominantly butanes from the laden solvent;   b. supplying the laden solvent as a bottoms stream from the absorber to a desorber, the bottom of which is at an elevated temperature relative to the bottom of the absorber, and in which the butenes are separated from the solvent to obtain at the top of the desorber a stream enriched in butenes, wherein the solvent, at least partially freed of butenes, is collected in a liquid collector of the desorber, passed through a desorber evaporator, and then passed into the bottom of the desorber below the liquid collector to outgas any butenes remaining in the solvent, and wherein the solvent is subsequently recycled as a bottoms stream from the desorber to the absorber;   wherein heat of the solvent withdrawn as the bottoms stream of the desorber is used for heat integration by employing the heat of the solvent in at least one respective heat exchanger for preheating the laden solvent passed to the desorber, for evaporation in the first evaporator and the second evaporator of the absorber, and for evaporation of the liquid C4-hydrocarbon stream,   wherein the absorber comprises a dividing plate in the absorber bottom to divide the absorber bottom into a first segment and a second segment, and the two-stage evaporation is performed such that the laden solvent collected in the liquid collector is run through the first evaporator, and passed to the first segment, and the laden solvent from the first segment is run through the second evaporator, and flashed into the second segment, from which the bottoms stream from the absorber is then withdrawn,   wherein the desorber evaporator comprises a heat exchanger and the heat for evaporation is provided by heat transfer with a suitable heat transfer medium, and   wherein the suitable heat transfer medium is heating steam, wherein the heating steam undergoes at least partial condensation in the heat exchanger, thus generating a hot condensate at a pressure of 10 to 20 bar absolute, and a temperature of 150° C. to 200° C., which is passed to a condensate container.   
     
     
         2 . The process according to  claim 1 , wherein the solvent is N-methyl-2-pyrrolidone (NMP). 
     
     
         3 . The process according to  claim 1 , wherein the solvent contains water and a water content is between 1% and 10% by weight. 
     
     
         4 . The process according to  claim 3 , wherein the water content is between 4% and 9% by weight. 
     
     
         5 . The process according to  claim 1 , wherein a stream enriched in butanes relative to the C4-hydrocarbon stream is obtained at the top of the absorber. 
     
     
         6 . The process according to  claim 1 , wherein the stream enriched in butenes obtained at the top of the desorber additionally contains water originating from the solvent. 
     
     
         7 . The process according to  claim 6 , wherein the stream enriched in butenes is withdrawn at the top of the desorber and subjected to a condensation, wherein a water-containing stream and a butene-containing product stream are condensed out and separated from one another. 
     
     
         8 . The process according to  claim 7 , wherein the butene-containing product stream obtained from the condensation has a butene content of at least 70% by weight, based on a total composition of the butene-containing product stream. 
     
     
         9 . The process according to  claim 8 , wherein the butene-containing product stream has a butene content of at least 86% by weight, based on the total composition of the butene-containing product stream. 
     
     
         10 . The process according to  claim 1 , wherein a temperature in the bottoms stream of the absorber, which is passed to the desorber, is between 70° C. to 130° C. 
     
     
         11 . The process according to  claim 1 , wherein a temperature in the bottom of the desorber is between 120° C. and 200° C. 
     
     
         12 . The process according to  claim 1 , wherein a head pressure in the desorber is between 1 and 6 bar absolute. 
     
     
         13 . The process according to  claim 1 , wherein the preheating of the laden solvent passed to the desorber is performed in two stages, wherein a first heat transfer to the solvent is effected in a heat exchanger and a second heat transfer to the solvent is effected in a kettle evaporator. 
     
     
         14 . The process according to  claim 1 , wherein the pressure in the condensate container is lower than in a heating steam side of the heat exchanger, thus causing a portion of the heating condensate to be re-evaporated, as a result of which a combined steam is obtained as low pressure steam. 
     
     
         15 . The process according to  claim 14 , wherein the heating steam for the desorber evaporator is provided using a steam ejector supplied with high pressure or medium pressure steam and the low pressure steam obtained in the condensate container. 
     
     
         16 . The process according to  claim 1 , wherein the bottom of the desorber is at a lower pressure relative to the bottom of the absorber. 
     
     
         17 . The process according to  claim 1 , wherein the first evaporator is a once-through evaporator. 
     
     
         18 . The process according to  claim 1 , wherein the second evaporator is a forced-circulation evaporator. 
     
     
         19 . A process for separating butenes from a C4-hydrocarbon stream which contains at least butenes, n-butane, and isobutane, by extractive distillation with a solvent, the process comprising:
 a. at least partially evaporating a liquid C4-hydrocarbon stream in a feed evaporator, to supply an absorber with a gaseous C4-hydrocarbon stream, supplying the solvent to the absorber above the gaseous C4-hydrocarbon stream and contacting therein the gaseous C4-hydrocarbon stream and the solvent with one another to transfer predominantly butenes from the gaseous C4-hydrocarbon stream to the solvent, to obtain a laden solvent, wherein the laden solvent is collected in a liquid collector and subjected to a two-stage evaporation in a first evaporator and a second evaporator to outgas predominantly butanes from the laden solvent;   b. supplying the laden solvent as a bottoms stream from the absorber to a desorber, the bottom of which is at an elevated temperature relative to the bottom of the absorber, and in which the butenes are separated from the solvent to obtain at the top of the desorber a stream enriched in butenes, wherein the solvent, at least partially freed of butenes, is collected in a liquid collector of the desorber, passed through a desorber evaporator, and then passed into the bottom of the desorber below the liquid collector to outgas any butenes remaining in the solvent, and wherein the solvent is subsequently recycled as a bottoms stream from the desorber to the absorber;   wherein heat of the solvent withdrawn as the bottoms stream of the desorber is used for heat integration by employing the heat of the solvent in at least one respective heat exchanger for preheating the laden solvent passed to the desorber, for evaporation in the first evaporator and the second evaporator of the absorber, and for evaporation of the liquid C4-hydrocarbon stream,   wherein the absorber comprises a dividing plate in the absorber bottom to divide the absorber bottom into a first segment and a second segment, and the two-stage evaporation is performed such that the laden solvent collected in the liquid collector is run through the first evaporator, and passed to the first segment, and the laden solvent from the first segment is run through the second evaporator, and flashed into the second segment, from which the bottoms stream from the absorber is then withdrawn, and   wherein the preheating of the laden solvent passed to the desorber is performed in two stages, wherein a first heat transfer to the solvent is effected in a heat exchanger and a second heat transfer to the solvent is effected in a kettle evaporator.